In connection with the manufacture of papermaking pulp, various steps in the process include material being transported in the form of chips or pulp. In certain parts of the process the material must be fed uniformly from bins for storage and/or treatment of the material to a subsequent treatment step. The problem in this case is to bring about a uniform flow through the bin. The material can adhere to the walls of the bin and thereby cause arching and/or ratholing and/or the material can move at different speeds in different parts of the cross-section of the bin. The feed of the material through the bin can thus be stopped, or the retention time of the material in the bin can vary. This is particularly unfavourable when the material is subjected to some kind of treatment in the bin, for example preheating with steam or treatment with chemicals. In this case it is wished that the treatment is made during a predetermined time, which is difficult if different parts of the material move in different ways.
Many different solutions of the above problem have been suggested.
U.S. Pat. No. 2,943,752 from 1960 disclose an early chip bin design for bulk feed with a rectangular lower outlet with a transition part down to the lower outlet that promote an even flow through the chip bin as such. This chip bin avoids the extensive welding work necessary to make the transition part. However, directly at the outlet end there is still a single screw feeding the bulk material sidewise, which screw hinders even flow out from the bin as the screw is filled primarily at the distant end as seen from the outlet.
EP 0 742 854 discloses among others a similar solution, but with one screw. Hence, the same problem with filling the screw from the end of the screws is at hand.
U.S. Pat. No. 4,958,741 discloses a bin with an upper part having a circular upper edge and an oval lower edge, where the length of the oval is equal to the diameter of the circle, and with a lower part having an oval upper edge and a circular lower edge. This chip bin is expensive to manufacture as a lot of welding work is necessary for each of the wall segments making up the chip bin convergence.
U.S. Pat. No. 6,336,573 discloses a bin with a cylindrical upper part and a lower part with an upper circular edge and a lower oval edge, where the length of the oval is equal to the diameter of the circle. Feeding out from the bin is made with a screw. Several proposals are mentioned addressing the problem with filling the screw evenly from the whole chip bin, and thus trying to decrease the disadvantages with using a screw. The proposals includes varying shaft diameter, flight diameter and varying pitch of the screw so as to provide a substantial uniform flow along the slot outlet.
SE 511 519 (=U.S. Pat. No. 6,250,514) and U.S. Pat. No. 6,089,417 shows more complicated solutions of bins per se resulting in a chip bin that is expensive to manufacture as a lot of welding work is necessary for each of the wall segments making up the chip bin convergence.
U.S. Pat. No. 7,178,698 discloses a bin with an upper cylindrical part and a lower part having a circular upper edge and a rectangular lower edge, where the length of the long side of the rectangle is equal to the diameter of the circle. Feeding out from the bin is made with two counter rotating screws, feeding towards a central circular outlet beneath the screws. However, the problem is that the screws are filled at the end of the screws emptying the chip bin from there, and at the central outlet the screw may not empty the bin in equal volume from the centre of the bin as the screw is already filled.
The problem with all these chips bins lies in the usage of screw conveyers that pick up bulk material close at wall of chip bin outlet and thus empties the chip bin from that end of screw, but not promoting an even flow through the entire chip bin volume. Even if the bin design per se is promoting even flow downwards, the screw conveyers prevents even flow as they are filled at the distant end as seen from the subsequent outlet.
In following description is the expression rotary pocket feeder used which is a conventional machine per se in feeding wood chips to a digester. Another name for the rotary pocket feeder is the star feeder. In a rotary pocket feeder are a number of pockets formed between axially oriented vanes on a revolving shaft, feeding comminuted material, for example chips from an upper inlet end to a lower outlet end after a rotation of about 180 degrees. The pockets are simply filled by gravity, and are emptied by gravity. Rotary pocket feeders have been used in conventional piping for feeding chips to digesters. In conventional continuous cooking systems has one rotary pocket feeder, i.e. chip meter, been used for chip volume measurements after the chip bin and a following rotary pocket feeder, i.e. low pressure feeder, has been used as a low pressure sluice feeder ahead of the steaming vessel. These rotary feeders are described more in detail in Chemical Pulping Book 6A (ISBN 952-5216-06-03), 1969, in chapter 6, section 3.2 “Chip metering and feeding into process pressure”, on pages A522-A514. However, these rotary pocket feeders, especially the chip meter, have had either a circular inlet or have been fed by a screw conveyer from the chip bin, and thus have had no impact on an uniform retention time of chips in the chip bin.